Subject of the invention is an adjuvant for opening up and depilation that is used in the manufacturing process known as liming of the untanned corium stripped of the epidermis and subcutaneous connective tissue, the so-called pelt.
As is known the overall leather manufacturing process consists of three main cycles viz. beamhouse, tanning and dressing.
In the beamhouse the salted and dried hides are first restored to the hydrated condition of the green hide by soaking. Then follows a treatment with alkaline baths for opening up and depilating the hides. Depilation and opening up are generally carried out in a single processing step, the liming process. Use is also made of fermentative depilation processes and processes in which the hairs are completely destroyed by the effect of keratolysing agents. Swelling and alkaline reaction are finally eliminated again by neutralisation. In this condition the hide is known as pelt.
The means used for the removal of the hairs, the epidermis and the hair roots generally depend on whether it is intended to chemically destroy the hairs or to save them. If the hairs are of little value, chemical depilatories are applied directly to the hair side of the hides, whereby the hair shafts are destroyed or at least more or less attacked. Normally a calcium suspension containing sulphide is used for this purpose. Industry practice is to carry out this treatment (liming) in large drums with agitation. The calcium suspension affects not only the hairs and the hair roots, but also the leather-building hide substance. This change of the hide substance, liming, is an essential component of the tanning process. To achieve optimal liming effect, little or no consideration is often given to hair saving even in cases where the hairs would be suitable for further use.
During the liming process the hide substance is in the first place opened up by alkaline swelling i.e. its structure is loosened resulting in greater porosity of the leather and special softness or stretch which is for instance desired for gloving leather. Moreover, some accompanying proteins of the fibrillar connective tissue are dissolved out. Duration and intensity of the liming can only be determined empirically in each case, because the various hides react quite differently to the liming process. Too much swelling can cause a xe2x80x9cloosexe2x80x9d or xe2x80x9crunningxe2x80x9d grain i.e. the top skin layer, containing the hair root pores, empty sweat glands and a ramified system of blood vessels comes loose from the compact fibre layer underneath when the leather flexes and forms creases. Liming is understood to include depilation and opening up of the hide, because they are often combined in one process. However, in most cases the two processes only overlap partly, because usually depilation and/or hair destruction are carried out first followed by the opening up of the hide substance. Just as in the depilation process, many individual factors also work together in the opening process. The liming chemicals must above all attack the keratin of the hairs and the epidermis without destroying the collagen of the hide. The keratin can be removed by reduction and hydrolysis. Hydrolytic bases for this purpose are e.g. calcium hydroxide, hydrated lime [with 80-96% Ca(OH)2] and caustic soda solution. Sodium sulphide, sodium hydrogen sulphide and calcium hydrogen sulphide have a reducing effect. The action of these substances also destroys the epidermis so that this layer together with the hairs can be easily removed with a blunt unhairing knife or by machine. The same as in depilatories the sodium sulphide has a reducing effect on the sulphurous amino acids of the keratin. After that the caustic lime can attack more easily and cause hydrolytic decomposition of the prekeratins in the basal cell layer of the epidermis.
After the hairs and the epidermis have been removed, the flesh and fat remnants adhering to the underside of the subcutaneous connective tissue are taken off with the sharp fleshing knife or the cylinder fleshing machine. The untanned corium without epidermis and subcutaneous connective tissue contains in moist condition 60 to 80% water and fat, the rest is ca. 98% collagen.
From the European patent application EP 0 728 844 we already know a multifunctional leather processing agent for the leather manufacture in the beamhouse. This agent contains a solution of proteolytic and lipolytic enzymes, molasses and hydrotropes and possibly other dispersive, swelling-inhibiting, depilatory or calcium-dissolving additives. This agent serves above all for improving the rehydration and dirt removal in the soak, improving the depilation, inhibiting swelling in the lime pit and improving the surface cleaning of the hide in the bate. Although a combination preparation of this kind already provided advantages over the hitherto practised individual use of the enzymes mentioned, there still remained the problem of finding an improved and surer liming process in order to obtain a better leather quality.
Now it was found that this problem can be solved by an adjuvant for opening up and depilation of animal hides which in aqueous solution contains:
10 to 50 Percentage by weight of a polysaccharide solution
5 to 25 Percentage by weight of a mercaptocarboxylic acid or of one of its salts and
1 to 10 Percentage by weight of a mercaptoalcohol or one of its alkali salts.
In the adjuvant as per invention molasses is used preferably as polysaccharide solution. The use of molasses in leather processing is already known as such. Molasses can be added during all operations in the beamhouse. The addition of molasses during liming is particularly useful, because it clearly improves the solubility of the hydrated lime in the float so that improved opening up of the hide is obtained. Although molasses generally deserves preference as a particularly economical saccharide solution, it is also possible to use other solutions containing water-soluble carbohydrates, e.g. sugar solutions with a sugar content of ca. 10 to 70 percentage by weight.
A further important constituent of the adjuvant as per invention is a mercaptocarboxylic acid or one of its salts, preferably thioglycolic acid. The use of these mercaptocarboxylic acids as keratolysing agents is already known. They are used extensively for hair treatment, but also as a depilatory. Thioglycolates also have been used in tanning for the unhairing of hides, but did not acquire much technical significance for this application.